Although information has been obtained on cell cycle characteristics of various human tumors, few studies have been designed to evaluate the changes that take place in tumor cell kinetics during the course of the human disease, and what effect chemotherapy and other forms of treatment have on the kinetics of tumor growth. The current proposal will attempt to obtain information on thymidine labeling indices and other cell kinetic parameters in-vivo, in a variety of human tumors prior to and after "perturbation" by conventional therapeutic procedures. The basic technique used will be one of intratumoral injection of tritiated thymidine followed by fine-needle aspiration or biopsy. Such studies can be carried out in many nodules or other tumor- involved sites simultaneously or serially following treatment. Short- term studies are planned following hormonal manipulations in hormone- responsive cancers, and long-term studies in other cancers treated with chemotherapy or radiotherapy. Such longitudinal studies should provide valuable information on factors influencing tumor cell growth at the clinical level. Moreover, the data obtained might be particularly helpful in providing explanations for features of the clinical course, in predicting therapeutic response or resistance, and ultimately in designing new treatment approaches.